Insights on the Design of a High End Tube Mic Preamp

At the Heart of the Edwards LE-10 Tube Mic Preamp are two high quality hand selected Vacuum Tubes. Each channel of audio uses two vacuum tubes, The 12AX7 and the EF86.

The first stage of amplification is very critical because microphones produce a low audio output. In that stage of amplification we use a 12AX7 tube. This tube is commonly used in the electronics industry because of its reliability, high quality and availability. What many people do not know about this tube, is that the 12AX7 is actually two tubes in one casing.

Tubes have filaments inside them, much like the filament that lights up an incandescent light bulb in your favorite lamp. The filaments can run on either AC or DC voltages. Our designer, Bill Bruins, chose to run the filaments of both the 12AX7 and the EF86 on DC voltages. Since AC voltages operate at 60hz, it is possible to add hum, which is also at 60hz, to an audio circuit by using AC voltages, and the associated wiring, to run the filaments. That problem is eliminated by operating the filaments with a regulated DC power supply.

The filament voltages for our tubes is 6.3 volts (either AC or DC). After much research and thought we have elected to run our filament voltages about four tenths of a volt lower, causing the vacuum tube preamp to operate relatively cool and giving longer life to the tubes. All of this without compromise to frequency response, signal to noise ratio, and all the other audio specifications.

The EF86 Vacuum Tube is legendary for its fine frequency response and quiet operation.

The EF86 tube is legendary for its fine frequency response and quiet operation. This tube is used, along with a Jensen transformer, to create a balanced 600 ohm output feeding into a Neutrik 3 pin XLR connector.

Tubes are noted for their warmth and musical sound. In every phase of design we have built circuitry that makes the tubes operate at their optimal values. Our tubes are user changeable. And because of the negative feedback circuitry designed into our mic preamp, the LE-10 works extremely well with a wide variety of tubes, ranging from moderately priced tubes to expensive ones.

You can find and read about Vacuum Tube Microphone Preamps all over the internet. But, hearing the sound of an Edwards LE-10 series preamp will quickly convince you that THIS Microphone preamp should be the next item in YOUR studio…………… The Mic Pre that will make your music come to life!